A pressing concern in anti-terrorism and homeland security is explosives detection. Most high explosives are nitro-substituted organic compounds. Typically, nitroaromatics, such as 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (2,4-DNT), are the primary military explosives and also the principle components in the unexploded landmines worldwide. Nitramines and nitrate esters (e.g. 3,5-trinitroperhydro-1,3,5-triazine (RDX) and pentaerythritol tetranitrate (PETN)) are the main components of highly energetic plastic explosives, such as C-4 (91% RDX) and Semtex (40-76% PETN). The demands of detecting hidden or buried explosives have led to an intense interest in low cost and ultrasensitive explosives detection techniques. Nitro explosives are also extremely sensitive to shock, friction and impact. Therefore, detection methods that permit contact-free analysis are desirable.
Several methods are currently available for the detection of explosives. The EPA approved standard technology for trace nitroaromatic and nitramine detection is EPA protocol SW-846 Method 8330a (http://www.epa.gov/osw/hazard/testmethods/sw846/online/8_series.htm). This method involves reverse-phase HPLC with UV detection. Detection technologies based on sensors have also been developed over the past decade. These technologies, which are typically patterned after the dog's or human's olfactory senses, can contain multiple receptors (or sensor arrays). Each vapor introduced to the electronic nose causes some or all of the sensor elements to respond differentially, producing unique response patterns that encode each vapor. In combination with pattern-recognition software, an artificial olfaction system is created that can recognize simple or complex odors. Another technique for nitro-explosive vapor detection involves ion mobility spectrometry (IMS) and gas chromatography coupled with mass spectrometry (GC-MS). Yet, these systems suffer from not satisfying one or more of the characteristics desired in a field detector, including portability, ease of use, real-time measurements, high sensitivity, selectivity for a range of compounds, high throughput capabilities, harmlessness to operator or bystanders, low cost, and applicability to solids, gases and liquids.
The stability of energetic materials is often assessed by their trigger linkage, which is generally the C—NO2 bond in nitro explosives. Consequently, a high nitro substitution has become an important characteristic and renders nitro explosives electrophilic. This characteristic also allows quenching of fluorophores through photoinduced electron transfer. Fluorescent conjugated polymers have been considered a leading structure in new explosives detection techniques due to their efficient exciton migration along the polymer chains. These polymers allow for fluorescence quenching over a long range by a single quencher-binding, or called “molecular wire” signal amplification. The application of these techniques to vapor detection of explosives, however, remains a challenge because most explosive materials have ultra-low volatility (e.g. the saturation vapor concentrations for HMX, RDX, and PETN are 0.1, 5 and 7 ppt, respectively), unfavorable reduction potential, and the lacking of conjugated electrons to engage in π-stacking.
Solid-state sensing materials have been used for vapor detection. The performance of most fluorescent sensory materials is limited by film thickness. The diffusion of analyte vapors in non-porous rigid films is slow. It has been reported that a spin-coated conjugated polymer film achieves its optimum quenching efficiency towards TNT vapor with an ultra-thin film (ca. 2.5 nm) and experiences a sharp drop in quenching efficiency at films thicker than 25 nm. See J. S. Yang, T. M. Swager, J. Am. Chem. Soc. 1998, 120, 5321. To reduce the dependence of sensing performance on film thickness, a sensor based on a highly porous nanostructure with a large surface-to-volume ratio, inherent high porosity, and easy accessibility of sensing materials is needed.